Known testing equipment tends to be slow and/or unreliable. For example, U.S. Pat. No. 4,649,740 discloses apparatus in which flexible packages are indexed through at least two test stations. At each station, a package stops and a load is applied by probe rods. Two successive stations apply the same load, and the effective thickness of a flexible package is measured in each case. The two thickness measurements are compared, and a difference is deemed to show a defective package. The idea is that a leaky package will be partly deflated at the first station, so that the effective thickness detected at the second station will be less. But we find that this is not reliable. For example, a grossly leaky package will be fully deflated at the first station. The reading at the second station will therefore be the same as at the first, and the package will be accepted as correct. Furthermore, the procedure is slow, owing to its reliance on static testing.